Exactly why tamron is a great option for you.
It's sharp enough for most things. Sure corners aren't the best 75mm f2.8 but are your portraits going to be in the corners or your landscapes shot at f2.8.
The AF is great, if you upgrade the body in future you'll enjoy the full eyeAF capability with this lens.
There are plenty of people shooting portraits with 24-70/2.8 zooms. On a budget or even otherswise this is simply a great all around lens which an 85mm prime or 50mm prime isn't.
Better to own one good all round lens than many mediocre ones or one really good specialist lens.
It's best to ignore the internet hyperbole which will tell you to buy 85GM or nothing else lol
If can't successfully shoot great portraits with the tamron it's not the gear that is the problem